Field of the Invention
This invention relates to direct positive, silver halide material which is capable of producing a positive image by the Herschel reversal process. More particularly this invention relates to Herschel type direct positive materials with reduced room or white light sensitivity.
Description of the Prior Art
Direct positive silver halide materials based on the Herschel effect use a reversal phenomenon by which fogging nuclei in certain silver halide emulsions are bleached or destroyed by radiation, usually long wavelength, visible light or infrared radiation. It is also well known to enhance the sensitivity to bleaching of such direct positive materials by incorporating desensitizers into the fogged emulsion to increase speed and/or change spectral sensitivity. Desensitizers described in the prior art which enhance Herschel bleaching include heterocyclic compounds such as benzothiazole, quinoline, indolenine and benzotriazole having a nitro group attached to a benzene nucleus, and dyes such as Pinakryptol Yellow, Pinakryptol Green, and Phenosafranine as well as nitro substituted styryl dyes such as those disclosed in U.S. Pat. No. 2,669,515.
An advantage of the direct positive materials described above is that they can be handled for short periods of time in moderate room or white light. However, in the case of the more sensitive direct positive materials containing the above mentioned desensitizing dyes too much room light exposure results in low density images with high background fog. Such low density may result from spectrally sensitized bleaching due to shorter wavelength radiation, e.g., green, blue, and ultraviolet radiation of room light. High background fog presumably results from conventional room light induced latent image formation, i.e., the residual forward speed of the direct positive material. In U.S. Pat. No. 3,526,507 the use of tetraazaindine compounds is disclosed for the prevention of white light induced, low density images in autopositive reproduction materials but not for the reduction of white light induced background fog.